The role of interface in microbubble ozonation of aromatic compounds

Micro and nanobubbles-assisted advanced oxidation processes for water decontamination: The importance of interface reactions

Micro and nanobubbles (MNBs), as an efficient and convenient method, have been widely used in water treatment. Composed of gas and water, MNBs avoid directly introducing potential secondary pollutants. Notably, MNBs exhibit significant advantages through interface reactions in assisting AOPs. They overcome barriers like low mass transfer coefficients and limited reactive sites, and shorten the distance between pollutants and oxidants, achieving higher pollutant removal efficiency. However, there is a lack of systematic summary and in-depth discussion on the fundamental mechanisms of MNBs-assisted AOPs. In this critical review, the characteristics of MNBs related to water treatment are outlined first. Subsequently, the recent applications, performance, and mechanisms of MNBs-assisted AOPs including ozone, plasma, photocatalytic, and Fenton oxidation are overviewed. We conclude that MNBs can improve pollutant removal mainly by enhancing the utilization of reactive oxygen species (ROS) generated by AOPs due to the effective interface reactions. Furthermore, we calculated the electrical energy per order of reaction (EE/O) parameter of different MNBs-assisted AOPs, suggesting that MNBs can reduce the total energy consumption in most of the tested cases. Finally, future research needs/opportunities are proposed. The fundamental insights in this review are anticipated to further facilitate an in-depth understanding of the mechanisms of MNBs-assisted AOPs and supply critical guidance on developing MNBs-based technologies for water treatment.

The effect of pre-treatments on atrazine removal from source water by microbubble ozonation

Ozone-based advanced oxidation processes (AOPs) have emerged a promising avenue for water treatment, offering effective removal of micropollutants. Recent research underscores the potential of ozone microbubbles to enhance ozone mass transfer during water treatment, particularly when combined with pre-treatment steps. This study aimed to evaluate the efficacy of three different combined processes (chlorine/KMnO4/PAC pre-treatment followed by ozonation) in removing atrazine, a common micropollutant from natural source water. Results revealed that all combined processes achieved higher atrazine removal rates compared to individual pre-treatment or ozonation methods. Notably, the highest atrazine removal rates were observed under alkaline pH conditions, with treatment outcomes influenced by oxidant dose and pH levels. Among the combined processes, chlorine pre-treatment followed by ozonation emerged as the most effective approach, achieving a removal rate of 59.7% that exceeded the sum of individual treatments. However, this treatment efficacy was affected by water quality parameters, particularly the presence of organic matter and elevated ammonia nitrogen concentration (> 0.5 mg/L). This study highlights the potential for utilizing ozone micro/nanobubbles to enhance ozone mass transfer and offers valuable insights for optimizing the combined application of pre-treatment and ozonation strategies for efficient atrazine removal from natural water sources.

Microstructured gas-liquid-(solid) interfaces: A platform for sustainable synthesis of commodity chemicals

Gas-liquid-solid catalytic reactions are widespread in nature and man-made technologies. Recently, the exceptional reactivity observed on (electro)sprayed microdroplets, in comparison to bulk gas-liquid systems, has attracted the attention of researchers. In this perspective, we compile possible strategies to engineer catalytically active gas-liquid-(solid) interfaces based on membrane contactors, microdroplets, micromarbles, microbubbles, and microfoams to produce commodity chemicals such as hydrogen peroxide, ammonia, and formic acid. In particular, particle-stabilized microfoams, with superior upscaling capacity, emerge as a promising and versatile platform to conceive high-performing (catalytic) gas-liquid-(solid) nanoreactors. Gas-liquid-(solid) nanoreactors could circumvent current limitations of state-of-the-art multiphase reactors (e.g., stirred tanks, trickle beds, and bubble columns) suffering from poor gas solubility and mass transfer resistances and access gas-liquid-(solid) reactors with lower cost and carbon footprint.

Effects of micro/nano-ozone bubble nutrient solutions on growth promotion and rhizosphere microbial community diversity in soilless cultivated lettuces

Due to its high efficacy as a wide-spectrum disinfectant and its potential for the degradation of pollutants and pesticides, ozone has broad application prospects in agricultural production. In this study, micro/nano bubble technology was applied to achieve a saturation state of bubble nutrient solution, including micro-nano oxygen (O2 group) and micro-nano ozone (O3 group) bubble nutrient solutions. The effects of these solutions on lettuce physiological indices as well as changes in the microbial community within the rhizosphere substrate were studied. The application of micro/nano (O2 and O3) bubble nutrient solutions to substrate-cultured lettuce plants increased the amount of dissolved oxygen in the nutrient solution, increased the lettuce yield, and elevated the net photosynthetic rate, conductance of H2O and intercellular carbon dioxide concentration of lettuce plants. Diversity analysis of the rhizosphere microbial community revealed that both the abundance and diversity of bacterial and fungal communities in the substrate increased after plant cultivation and decreased following treatment with micro/nanobubble nutrient solutions. RDA results showed that the microbial community in the S group was positively associated with EC, that in the CK and O2 groups exhibited a positive correlation with SC, and that in the O3 group displayed a positive correlation with CAT and POD. Overall, the implementation of micro/nanobubble generation technology in soilless substrates can effectively increase the lettuce growth and yield, and O3 had a more pronounced effect on lettuce yield and quality and the microbial community structure in the substrate than O2. Our study would provide a reference and theoretical basis for developing sustainable and green technology for promoting lettuce production and can be a promising alternative to conventional methods for improving crop yields.

PFAS removal from landfill leachate by ozone foam fractionation: System optimization and adsorption quantification

Landfills are the primary endpoint for the disposal of PFAS-laden waste, which subsequently releases PFAS to the surrounding environments through landfill leachate. Ozone foam fractionation emerges as a promising technology for PFAS removal to address the issue. This study aims to (i) assess the effectiveness of the ozone foam fractionation system to remove PFAS from landfill leachate, and (ii) quantify equilibrium PFAS adsorption onto the gas-water interface of ozone bubbles, followed by a comparison with air foam fractionation. The results show that ozone foam fractionation is effective for PFAS removal from landfill leachate, with more than 90 % long-chain PFAS removed. The identified operating conditions provide valuable insights for industrial applications, guiding the optimization of ozone flow rates (1 L/min), dosing (43 mg/L) and minimizing foamate production (4 % wettability). The equilibrium modelling reveals that the surface excess of air bubbles exceeds that of ozone bubbles by 20-40 % at a corresponding PFAS concentration. However, the overall removal of PFAS from landfill leachate by ozone foam fractionation remains substantial. Notably, ozone foam fractionation generates foamate volumes 2 - 4 times less, resulting in significant cost savings for the final disposal of waste products and reduced site storage requirements.

Influence of static pressure on toluene oxidation efficiency in groundwater by micro-nano bubble ozonation

Micro-nano bubble ozonation has been widely applied in the purification of drinking water due to its superior characteristics such as high mass transfer rate and long resistance time. However, its application in groundwater remediation is limited, partially due to the unclear effect of static water pressure on the oxidation efficiency. This study constructed a batch reactor to investigate the influence of static pressure on toluene oxidation by ozone micro-nano bubble water. To achieve constant pressure, weight was added above the mobile reactor roof, and the initial concentrations of toluene and dissolved ozone were 1.00 mg L-1 and 0.68 mg L-1 respectively. Experimental results demonstrated that as the static water pressure increased from 0.0 to 2.5 m, the average microbubble diameter decreased significantly from 62.3 to 36.0 μm. Simultaneously, the oxidation percentage of toluene increased from 40.3% to 58.7%, and the reaction rate between toluene and hydroxyl radical (OH·) increased from 9.3 × 109 to 1.39 × 1010 M-1 s-1, indicating that the shrinkage of micro-nano bubbles generated an abundance of OH· that quickly oxidized toluene adsorbed at the bubble interface. A greater enhancement of oxidation efficiency for nitrobenzene, as compared to p-xylene, was observed after the addition of 2.5 m water pressure, which verified the larger contribution of OH· under static pressure. Although the improvement of oxidation efficiency was reduced under acid and alkaline environments, as well as in practical groundwater matrices, the overall results still demonstrated the promising application of micro-nano bubble ozonation in groundwater remediation.

Ozone Micronano-bubble-Enhanced Selective Degradation of Oxytetracycline from Production Wastewater: The Overlooked Singlet Oxygen Oxidation

The efficient and selective removal of refractory antibiotics from high-strength antibiotic production wastewater is crucial but remains a substantial challenge. In this study, a novel ozone micronano-bubble (MNB)-enhanced treatment system was constructed for antibiotic production wastewater treatment. Compared with conventional ozone, ozone MNBs exhibit excellent treatment efficiency for oxytetracycline (OTC) degradation and toxicity decrease. Notably, this study identifies the overlooked singlet oxygen (1O2) for the first time as a crucial active species in the ozone MNB system through probe and electron paramagnetic resonance methods. Subsequently, the oxidation mechanisms of OTC by ozone MNBs are systematically investigated. Owing to the high reactivity of OTC toward 1O2, ozone MNBs enhance the selective and anti-interference performance of OTC degradation in raw OTC production wastewater with complex matrixes. This study provides insights into the mechanism of ozone MNB-enhanced pollutant degradation and a new perspective for the efficient treatment of high-concentration industrial wastewater using ozone MNBs. In addition, this study presents a promising technology with scientific guidance for the treatment of antibiotic production wastewater.

Nanobubble Reactivity: Evaluating Hydroxyl Radical Generation (or Lack Thereof) under Ambient Conditions

Nanobubble (NB) generation of reactive oxygen species (ROS), especially hydroxyl radical (·OH), has been controversial. In this work, we extensively characterize NBs in solution, with a focus on ROS generation (as ·OH), through a number of methods including degradation of ·OH-specific target compounds, electron paramagnetic resonance (EPR), and a fluorescence-based indicator. Generated NBs exhibit consistent physical characteristics (size, surface potential, and concentration) when compared with previous studies. For conditions described, which are considered as high O2 NB concentrations, no degradation of benzoic acid (BA), a well-studied ·OH scavenger, was observed in the presence of NBs (over 24 h) and no EPR signal for ·OH was detected. While a positive fluorescence response was measured when using a fluorescence probe for ·OH, aminophenyl fluorescein (APF), we provide an alternate explanation for the result. Gas/liquid interfacial characterization indicates that the surface of a NB is proton-rich and capable of inducing acid-catalyzed hydrolysis of APF, which results in a false (positive) fluorescence response. Given these negative results, we conclude that NB-induced ·OH generation is minimal, if at all, for conditions evaluated.

Oxidation of Congo red by Fenton coupled with micro and nanobubbles

Dye wastewater is a kind of refractory organic wastewater. Fenton coupled with micro-nano bubbles (MNBs+FT) was used for the degradation of Congo red (CR), aiming at simplifying the organic pollutants degradation process and reducing the cost of the process. The optimum condition of Fenton alone, the outlet pressure of the cavitation process and different combinations on the degradation of CR dye wastewater were discussed in this study. The results showed that the degradation of CR (100 mg/L) could reach 94.4% by using the MNBs+FT at the pH of 7, which was 72% higher than that using Fenton oxidation alone and 79% higher than that using MNBs alone. Based on the same degradation efficiency, the traditional Fenton process alone required 8 times the dose of oxidants of these combination systems, and the synergy coefficient of MNBs+FT was up to 2.44. ESR analysis indicated that ·OH was the predominant active species during the degradation of CR and MNBs+FT improved the utilization efficiency of H2O2 and produced more ·OH. Besides, the MNBs+FT could extend the pH range of the high-efficiency oxidation reaction, and it could also keep a high degradation rate under neutral conditions, which eliminated the process of adjusting the pH and reduced the anti-corrosion requirements of the equipment. According to the economic analysis results, the total cost of treatment for the MNBs/FT was about 13% of the cost of only the Fenton process. This study provides a reference for the application of MNBs+FT systems in full-scale dye wastewater treatment.

Ozone micro-bubble aeration using the ceramic ultrafiltration membrane with superior oxidation performance for 2, 4-D elimination

Micro-bubble aeration is an efficient way to promote ozonation performance, but the technology is challenged by extensive energy cost. Here, a ceramic ultrafiltration membrane was used to achieve ozone micro-bubble (0-80 µm) aeration in a simple way at gaseous pressures of 0.14-0.19 MPa. Compared with milli-bubble aeration, micro-bubble aeration increased the equilibrium aquatic O3 concentrations by 1.53-3.25 times and apparent O3 transfer rates by 3.12-3.35 times at pH 5.0-8.0. Consequently, the •OH yield was 2.67-3.54 times via faster O3 transfer to the aquatic solution followed by decomposition rather than interfacial reaction. Ozone micro-bubble aeration outperformed milli-bubble aeration, with the degradation kinetics of 2,4-D being 3.08-4.36 times higher. Both O3-oxidation and •OH oxidation were important to the promotion with the contributions being 35.8%-45.9% and 54.1%-64.2%, respectively. The operational and water matric conditions influenced the oxidation performance via both O3 oxidation and •OH oxidation, which is reported for the first time. In general, the ceramic membrane offered a low-energy approach of ozone micro-bubble aeration for efficient pollutant degradation. The O3 oxidation and •OH oxidation were proportionally promoted by ozone micro-bubble due to O3 transfer enhancement. Thus, the promotive mechanism can be interpreted as the synchronous enchantment on ozone exposure and •OH exposure for the first time.

The catalytic oxidation process of atrazine by ozone microbubbles: Bubble formation, ozone mass transfer and hydroxyl radical generation

Ozone microbubbles have received increasing attention since they can produce hydroxyl radical (•OH) to decompose ozone-resistant pollutants. Besides, compared with conventional bubbles, microbubbles have a larger specific surface area and higher mass transfer efficiency. However, the research on the micro-interface reaction mechanism of ozone microbubbles is still relatively scarce. Herein, we systematically studied the stability of microbubbles, ozone mass transfer and atrazine (ATZ) degradation through multifactor analysis. The results revealed that bubble size was dominant in the stability of microbubbles, and gas flow rate played a major role in ozone mass transfer and degradation effects. Besides, the bubble stability accounted for the different effects of pH on ozone mass transfer in two aeration systems. Finally, kinetic models were built and employed to simulate the kinetics of ATZ degradation by •OH. The results revealed that conventional bubbles could produce •OH faster compared with microbubbles under alkaline conditions. These findings shed light on the interfacial reaction mechanisms of ozone microbubbles.

Pilot-scale advanced treatment of actual high-salt textile wastewater by a UV/O3 pressurization process: Evaluation of removal kinetics and reverse osmosis desalination process

Advanced oxidation processes (AOPs) such as ozonation and Fenton processes are widely used in the treatment of high-salt wastewater. The UV/O₃ pressurization process was designed and applied at the pilot-scale for treatment of actual high-salt textile wastewater. The UV/O₃ pressurization process achieved the highest decolorization (85 %) and chemical oxygen demand (COD_Cr, 43.2 %) removal efficiency at an O₃ dosage of 200 g·t^-1 and a pressure of 0.2 MPa. Compared to ordinary ozonation, the UV/O₃ pressurization process improved the solubility and gas-liquid mass transfer efficiency of O₃ in wastewater and generated a large number of O₃ microbubbles. Hydroxyl radical (·OH), superoxide radicals (O₂·^-) and single oxygen (¹O₂) all played a significant role on the removal of pollutants in wastewater during the UV/O₃ pressurization process. The reverse osmosis (RO) process was used to evaluate the effect of UV/O₃ pressurization and Fenton pre-oxidation processes on the desalination process as the last process in treating high-salt organic wastewater. The pre-oxidation processes improved the initial RO water flux. Compared with the Fenton process, the UV/O₃ pressurization process had less membrane fouling (thin fouling layer vs thick fouling layer), and final water flux (59.4 LMH) was higher than that of Fenton process (34.9 LHM). The total dissolved solids (TDS), Cl^- and SO₄^2- of the effluent from UV/O₃ pressurization process (37.2, 7.6 and 3.0 mg·L^-1) were better than that of Fenton process (65.7, 13.9 and 7.1 mg·L^-1). Therefore, the UV/O₃ pressurization process without secondary pollution is more suitable for the advanced treatment of high-salt organic wastewater than the Fenton process.

Efficiency and mechanism of ozonated microbubbles for enhancing the removal of algae and algae-derived organic matter

The effective control of eutrophication caused by algae blooms is still the focus of global attention. The traditional dissolved air floatation process for algae removal has a low adhesion efficiency between bubbles and algal cells and a low removal efficiency of organic pollutants. Aiming to address these defects, this study set up an ozone microbubble-enhanced air flotation experiment to explore the removal trends of algal cells and algal organic matter (AOM) pollution. In contrast to traditional air flotation, this approach targets the removal of various forms of AOM after algal cell damage. The highest removal rates of algal cells, extracellular microcystin (Mc), intracellular Mc-lr and total Mc-lr were 96.6%, 60.1%, 95.2% and 93.7%, respectively. Compared with the traditional process, the absorption rate and utilization rate of ozone were increased by 41.9% and 46.2%, respectively. The removal effect of AOM was also greatly improved, and ozone microbubbles enhanced the removal of aromatic protein-like substances and high-molecular-weight fulvic acid, humic acid and humic substances. The advantageous synergistic effect of ozone and microbubbles on algae removal was analyzed by exploring the enhanced air flotation removal mechanism of ozone microbubbles' enhanced air floatation removal. Good vacuole adhesion and strong oxidation caused by ozone microbubbles jointly guaranteed a good removal rate of AOM. The enhanced air flotation process with ozone microbubbles has high feasibility and a good effect, can effectively remove algal cells and algal pollutants, and has great potential in algal removal and control of water eutrophication.

Contemporary application of microbubble technology in water treatment

Microbubble (MB) technology constitutes a suite of promising low-cost technologies with potential applications in various sectors. Microbubbles (MBs) are tiny gas bubbles with diameters in the micrometre range of 10-100 μm. Along with their small size, they share special characteristics like slow buoyancy, large gas-liquid interfacial area and high mass-transfer efficiency. Initially, the review examines the key dissimilarities among the different types of microbubble generators (MBG) towards economic large-scale production of MBs. The applications of MBs to explore their effectiveness at different stages of wastewater treatment extending from aeration, separation/ flotation, ozonation, disinfection and other processes are investigated. A summary of the recent advances of MBs in real and synthetic wastewater treatment, existing research gaps, and limitations in upscaling of the technology, conclusion and future recommendations is detailed. A critical analysis of the energetics and treatment cost of combined approaches of MB technology with other advanced oxidation processes (AOPs) is carried out highlighting the potential applicability of hybrid technology in large-scale wastewater treatment.

Micro and nano bubbles promoted biofilm formation with strengthen of COD and TN removal synchronously in a blackened and odorous water

Blackening and odorization of rivers (BOR) distributed widely in urban cities with high density of human beings. Amounts of pollution control methods have been developed for treatment of these contaminated rivers. Among them, artificial aeration is an effective method for BOR treatment. As a novel developed aeration approach, Micro and nano bubbles (MNBs) takes advances of high specific surface area, high oxygen transfer, long retain time and interface effect. Thus, MNBs aeration was used in an anoxic-oxic (AO) process with traditional activated sludge methods to treat water of BOR in this study. A special designed reactor was made to allow both MNBs and macro bubbles aeration of which mode could be altered easily. The results revealed that MNBs improved removal of COD, NH₄⁺-N and TN distinctly in water of BOR. MNBs provided high dissolved oxygen and promoted the transformation from floc sludge to biofilm. Significant difference between the microbial community of MNBs and macro bubbles sludges was revealed by 16S rRNA amplicon sequencing. Function predictions of MNBs and macro bubbles sludges indicated MNBs enhanced nitrification and aerobic ammonia oxidation without negative impact on denitrification. Moreover, biofilm formed bacteria were enriched by MNBs aeration. This study demonstrated MNBs would be a great potential for the combination of activated sludge and biofilm to treat BOR.

Self-Accelerating Interfacial Catalytic Elimination of Gaseous Sulfur-Containing Volatile Organic Compounds as Microbubbles in a Facet-Engineered Three-Dimensional BiOCl Sponge Fenton-Like Process

The elimination of gaseous sulfur-containing volatile organic compounds (S-VOCs) by a microbubble-assisted Fenton-like process is an innovative strategy. Herein, we established a microbubble-assisted Fenton-like process to eliminate malodorous microbubble CH₃SH as representative gaseous S-VOCs, in which BiOCl nanosheets loaded on a three-dimensional sponge were exposed to (001) or (010) facets and induced Fenton-like interface reactions. Intriguingly, the microbubble-assisted Fenton-like process significantly removed 99.9% of CH₃SH, higher than that of the macrobubble-assisted Fenton-like process (39.0%). The self-accelerating interfacial catalytic mechanism was in-depth identified by in situ ATR-FTIR, PTR-TOF-MS, EPR, and DFT computational study. The extraordinary elimination performance of microbubble-assisted Fenton-like process lies in the enhancing dissolution/mass transfer of gaseous CH₃SH in the gas/liquid phase and the tight contact between CH₃SH-microbubbles and 3D-BiOCl sponge due to the low rising velocity (0.13 mm s^-1) and negative charge (-45.53 mV) of CH₃SH-microbubbles, as well as the effective generation of ¹O₂ by activating the enriched dissolved oxygen in CH₃SH-microbubble via effective electron-polarized sites on 3D-BiOCl sponge. Furthermore, CH₃SH-microbubbles transferred electrons to H₂O₂ through electron-rich oxygen vacancy centers of the 3D-BiOCl sponge to generate more •OH, thus achieving excellent elimination performance. Overall, this study demonstrates the enhanced self-accelerating interfacial catalytic elimination by S-VOC microbubble and provides the underlying mechanisms.

Degradation of Methyl Orange by ozone microbubble process with packing in the bubble column reactor

The performance of the ozone microbubble(MB) process for the degradation of Methyl Orange (MO) in a bubble column reactor with added packing was investigated. The highest decolorization efficiency of 96.04% was achieved by the ozone MB process with packing, which was 10.17% and 62.02% higher than that of the ozone MB process without packing and the ozone millimeter bubble(MLB) process, respectively while keeping other operating parameters the same. In addition, the saturation gas holdup, ozone mass transfer coefficient, and decolorization rate constant of the ozone MB process with packing were 15.32%, 0.260 min^-1, and 0.027 min^-1, respectively, which were much better than those of the ozone MB process without packing and the ozone MLB process. The study also suggested that within a certain porosity range, the types of packings did not affect the performance of the ozone MB process in the degradation of MO. Moreover, the optimum operating conditions were initial concentration of MO of 30 mg/L, initial pH of 3, circulating liquid flow of 75 L/h, and ozone dosage of 0.56 mg/L. The decolorization efficiency was 99.28% within 120 min.

Implications of hydrogen peroxide on bromate depression during seawater ozonation

The presence of hydrogen peroxide (H₂O₂) in ozonation process can resist the formation of carcinogenic bromate (BrO₃¯) efficiently, and the bromate depression is closely related with background water qualities, especially in high bromide-containing seawater. In this study, the freshwater and seawater were selected to investigate the effects of H₂O₂ on ozone (O₃) decomposition kinetics, bromide transformation and bromate depression, and the evolutions of BrO₃¯ under different scavengers were explored to speculate the primary bromate formation pathways. The results showed that the initial O₃ half-live period (t_1/2-O₃) in seawater was only one-sixth of that in freshwater, and its attenuation rate increased analogously with the increase of H₂O₂ concentration in both freshwater and seawater. The H₂O₂ could promote the formation of BrO₃¯ via hydroxyl radical (•OH) based bromate pathways, nevertheless higher concentration of H₂O₂ facilitated the reduction of HOBr/OBr¯ back to Br¯, resulting in 87.0% and 73.2% of BrO₃¯ retardment in freshwater and seawater, respectively. The suppression ratios of BrO₃¯ were up to 48.4% and 35.3% in freshwater with the addition of •OH and •O₂¯ scavengers, and the corresponding depressions in seawater decreased to 35.3% and 12.7%, indicating that •OH was dominant on bromate formation when the concentration of residual ozone was adequate to generate some bromine intermediates, meanwhile H₂O₂ and •O₂¯ functioned as the key reductants for bromate depression. Based on these results, the Br¯ transformation mechanisms via O₃, •OH, H₂O₂, and •O₂¯ reactions were speculated, and the feasibility of H₂O₂-ozonation was verified for the treatment of high Br¯-containing seawater.

Performance of microbubble ozonation on treated tropical peat water: Effects on THM4 and HAA5 precursor formation based on DOM hydrophobicity fractions

The hydrophobicity properties of dissolved organic matter (DOM) found in tropical peat water has an impact on the formation of carcinogenic DBPs such as trihalomethanes-4 (THM4) and haloacetic acids-5 (HAA5). This study was conducted to determine the effect of microbubble ozonation on changes in DOM fraction and its effect on the formation of THM4 and HAA5. Alum coagulation and activated carbon adsorption were carried out to reduce the DOM concentration before microbubble ozonation. Microbubble ozonation was carried out at acidic (pH 5.5), neutral (pH 7) and alkaline (pH 8.5) conditions to determine the effect of pH. Coagulation and adsorption of activated carbon were successful in reducing the presence of the hydrophobic acid fraction (HPOA) in peat water completely, but the transphilic (TPH), charged hydrophilic (HPIC) and neutral hydrophilic (HPIN) fractions remained in the water. Microbubble ozonation succeeded in decreasing the presence of TPH fraction but increased the formation of HPIC and HPIN. The degradation of the TPH fraction resulted in reduced formation of chlorinated THM4 and HAA5 (C-THM4 and C-HAA5). On the other hand, the formation of HPIC and HPIN fractions increased the formation of brominated THM4 and HAA5 (B-THM4 and B-HAA5) after the final chlorination process.

Can bulk nanobubbles be stabilized by electrostatic interaction?

It has been suggested that electrostatic stress arising from charges accumulated at the surface of nanobubbles might balance Laplace pressure leading to their stability. This mechanism has been widely discussed in the nanobubble field for the past decade. However, the stress in the diffusive double layer was overlooked when calculating the electrostatic effect in previous theories. In this communication, we recalculated this effect using the classical double layer theory. Combined with experimentally measured zeta potential, we find that the ratio of electrostatic pressure to Laplace pressure is much less than 10-2, which suggests that electrostatic interaction may not be the main factor for stabilizing bulk nanobubbles.

The application of UV/O3 process on ciprofloxacin wastewater containing high salinity: Performance and its degradation mechanism

The increasing discharge of high-salinity organic wastewater has drawn much concern. This work investigated the degradation and mineralization of ciprofloxacin (CIP) in high-salinity wastewater by ozonation coupled with ultraviolet irradiation (UV). After coupling with UV, the removal efficiency of CIP was increased insignificantly (maximum 5.0%), while the dissolved organic carbon (DOC) removal in CIP wastewater (CW) was enhanced dramatically to 91.4% as compared with independent O₃ (37.5%). The reactive oxygen species (ROS) were identified as singlet oxygen (¹O₂) and superoxide anion radical (O₂^-•)·through electron paramagnetic resonance (EPR) and quenching experiments, among which ¹O₂ predominated in the UV/O₃ process. The existence of salt (Na₂SO₄ or NaCl) accelerated the mass transfer of O₃ at the gas-liquid interface, thus CIP removal was promoted in UV/O₃/SO₄^2- system. However, excessive Cl^- inhibited the removal efficiency of DOC in CW owing to its consumption of O₃. CIP degradation decreased as pH increased in non-salinity and UV/O₃/SO₄^2- system, which proved the direct reaction occurred between CIP and O₃. On the contrary, the O₃ mass transfer increased with increasing pH, hence the elimination of DOC in CW was promoted in UV/O₃/Cl^- system. Volatile organic compounds (VOCs) were detected from tail gas, but the toxicity estimation indicated the toxicity of products was similar or less than that of CIP. Overall, this work is meaningful for the practical application of UV/O₃ process in the high-salinity industry.

Collective Dynamics of Bulk Nanobubbles with Size-Dependent Surface Tension

It has been suggested that irreversible adsorption at the gas/liquid interface of bulk nanobubbles will reduce the Laplace pressure, leading to their stability. However, most previous studies have focused on the stability of individual nanobubbles. Bulk nanobubbles are polydispersed suspensions, and gas molecules can diffuse between bubbles, leading to their collective dynamics, which may be crucial to understanding their formation process and stability. In this study, we proposed a mean-field theory for computing the evolution of the size-distribution function of bulk nanobubbles with size-dependent surface tension. We applied this theory to investigate the evolution of bulk nanobubbles with insoluble surfactants pinned at their gas/water interface. The results show that Ostwald ripening can be suppressed when enough surfactants are adsorbed. Bulk nanobubbles can be produced by the shrinkage of microbubbles in an air-saturated solution. The mean stable size is controlled by the amount of surfactants and the initial microbubble concentration; these predictions are qualitatively consistent with the experimental results of micro/nanobubbles produced using the microfluidic method.

Degradation of Bisphenol A by ozonation in rotating packed bed: Effects of operational parameters and co-existing chemicals

Bisphenol A (BPA), a typical endocrine disrupting chemical, widely exists in water and threatens human health. The degradation of BPA by ozone in water is limited by the gas-mass transfer due to the low solubility of ozone. In this study, a rotating packed bed (RPB) was employed to create a high gravity environment to intensify the ozone mass transfer and BPA degradation. The effects of operational parameters (rotation speed of RPB, pH of the solution, ozone concentration, BPA concentration, gas volumetric flow rate and liquid volumetric flow rate) on BPA degradation efficiency and overall volumetric mass transfer coefficient of ozone were investigated. The results show that RPB effectively promoted the ozone mass transfer and BPA degradation and can be used for the ozonation of micropollutants that have fast reaction rates with ozone. Quenching experiments suggest that both ozone and HO∙ participated in BPA degradation from acidic to alkaline environments. In addition, the effects of co-existing chemicals on BPA degradation efficiency were studied. The addition of H₂O₂ or Cl^- had no obvious impact on BPA degradation; the addition of HCO₃^- is beneficial for BPA degradation while the addition of fulvic acid suppressed the degradation. These results indicate that the pH value, which affects the reaction rate between ozone and BPA, is a major factor to be considered during the ozonation of BPA in RPB.

Degradation of organic pollutants by ultraviolet/ozone in high salinity condition: Non-radical pathway dominated by singlet oxygen

In this work, the combined ultraviolet ozone process (UV/O₃) was applied for organic contaminant (Reactive Blue K-GL, RB) degradation in high salinity. The degradation rates of RB in both O₃ and UV/O₃ systems were enhanced by NaCl (the k increased from 0.080 to 0.116 to 0.132 and 0.267 min^-1 respectively), while mineralization rate varied at different salt conditions. In addition, UV irradiation promoted the degradation efficiency of RB with the presence of salt. Singlet oxygen (¹O₂) was the primary active species in the UV/O₃ system. The quenching experiments and signal intensity of ¹O₂ corresponded well to the mineralization of RB. Under conditions of high salinity and high pH, O₃ has high mass transfer coefficient (k_La, 3.303 min^-1) and self-decomposition (k_d, 0.600 min^-1), which further promoted the formation of ¹O₂ for mineralization of RB. Furthermore, UV/O₃ system was efficient in real textile wastewater treatment (COD_Cr removal rate 91.7% and decolorization rate 98.7%).

Short- and long-term effects of copper on anammox under gradually increased copper concentrations

This study aims to determine both short- and long-term response of enriched anammox culture to Cu. Assessment of short-term inhibition is based both on total applied Cu concentration and potential bioavailable fractions like intracellular, surface-bound, soluble and free Cu ion. The half maximal inhibitory concentration (IC50) values for total applied, soluble, intracellular and cell-associated concentrations were determined as 4.57 mg/L, 1.97 mg/L, 0.71 mg/L, 1.11 mg/L, respectively. Correlation between the surface-bound fraction of Cu and inhibition response was weak, suggesting that Cu sorbed to biomass was not directly responsible for the effects on anammox activity. There was a disparity between the results of short- and long-term experiments in terms of inhibition threshold concentration (i.e. short-term IC50 = 4.57 mg/L vs long-term IC50 = 6.74 mg/L). Candidatus Kuenenia (59.8%) and Candidatus Brocadia (40.2%) were the two main anammox genera within the initial biomass sample. One of the most interesting finding of the study is the demonstration that a complete wash-out of C. Brocadia genus at an applied Cu concentration of 6.5 mg/L. This strongly indicates that C. Brocadia were not able to tolerate high copper concentrations and all nitrogen conversion was carried out by C. Kuenenia during the Cu exposure period.

FeOx@GAC catalyzed microbubble ozonation coupled with biological process for industrial phenolic wastewater treatment: Catalytic performance, biological process screening and microbial characteristics

Phenolic compounds are common ccontaminants in industrial effluents. In this study, a combined catalytic microbubble ozonation and biological process was developed and applied for efficient industrial phenolic wastewater (PWW) treatment. Catalytic activity of an iron-oxides (FeO_x) doped granular activated carbon (GAC) catalyst (FeO_x@GAC) in microbubble ozonation for PWW treatment was investigated. The results demonstrated that the FeO_x@GAC catalyzed microbubble ozonation (O₃/FeO_x@GAC) obtained significantly higher reaction rate constant (k₁ = 0.023 min^-1) in TOC removal compared to the bare GAC catalyzed microbubble ozonation (O₃/GAC, k₁ = 0.013 min^-1) and ordinary microbubble ozonation (k₁ = 0.008 min^-1). Destruction rate constant of phenolic compounds (k₂) was improved from 0.014 min^-1 (ordinary microbubble ozonation) to 0.025 min^-1 (O₃/FeO_x@GAC). The 60-min pretreatment of PWW by O₃/FeO_x@GAC process enhanced BOD₅/COD ratio from 0.31 to 0.76 and reduced the acute bio-toxicity by 79.2%. Screening and characterization of biological post-treatment processes were conducted among activated sludge process (ASP), up-flow anaerobic sludge blanket (UASB) and membrane bioreactor (MBR). UASB and ASP showed limited phenolic compounds removal of 35.4% and 57.0% with lower bio-toxicity resistance than MBR (94.9% phenolic compounds removal). The combined process O₃/FeO_x@GAC-MBR was thus developed and achieved high COD removal (98.0%) and phenolic compounds degradation (99.4%). PWW pretreatment by O₃/FeO_x@GAC process decreased membrane fouling rate of MBR by 88.2% by reducing proteins/polysaccharides accumulation in both extracellular polymeric substances and soluble microbial products. 16S rRNA high-throughput sequencing revealed the predominance of phylum Proteobacteria, class Alphaproteobacteria and genera Mycobacterium, Gordonia, Pedomicrobium & Defluviimonas in biological PWW treatment bio-systems. Pearson correlation coefficient and ANOVA analysis verified that Mycobacterium possessed high bio-toxicity resistance and was the main contributor to the biodegradation of phenolic compounds.

Degradation of recalcitrant organic matter in SAARB leachate by a combined process of coagulation and catalytic ozonation

A combined coagulation and γ-Al₂O₃ catalytic ozonation process was used to treat semi-aerobic aged refuse biofilter (SAARB) effluent from treating mature landfill leachate. First, the coagulant providing the best pretreatment performance was selected. Then, the coagulated SAARB leachate was further treated in an optimized γ-Al₂O₃-catalyzed ozonation process. Characteristics of the γ-Al₂O₃-catalyzed ozonation process were determined, and a reaction mechanism was proposed. FeCl₃ provided the best treatment efficiency (chemical oxygen demand (COD) removal of 65.8%, absorbance at 254 nm (UV₂₅₄) removal of 68.55%, and color number (CN) removal of 79.4%). Under optimized O₃ dosage (18.92 mg/min) and γ-Al₂O₃ dosage (10 g/L), efficiencies of removing COD, UV₂₅₄, and CN were 54.3%, 82.9%, and 95.9%, respectively, at 30 min. In addition, spectral analysis indicated that fulvic-like substances in ultraviolet and visible regions were effectively degraded in the γ-Al₂O₃-O₃ process and some smaller organic products were produced. Characterization of γ-Al₂O₃ showed that γ-Al₂O₃ was relative stable; its morphology and constituent elements did not change much after reaction. In addition, ozonation capacity was enhanced by heterogeneous catalytic effects of γ-Al₂O₃. The combined coagulation and γ-Al₂O₃ catalytic ozonation process was proven to be an efficient treatment method for removing bio-refractory organic matter contained in SAARB leachate.

The progressive steps for TPH stripping and the decomposition of oil refinery sludge using microbubble ozonation

Total petroleum hydrocarbons (TPH) in activated petroleum waste sludge (PWS) hindered the disintegration of sludge, and microbubble ozonation (MB-O₃) was explored to separate the TPH and solids particle, enhance the decomposition of PWS, and improve the efficiency of ozonation. The maximum solubilization of PWS reached to approximately 41.9% at an ozone dose of 5.40 gO₃/gTS, two times higher than the control one. The ozone mass transfer coefficient of k_La increased from 0.1101 min^-1 to 0.2293 min^-1 in MB-O₃, resulting in the formation of a higher concentration of 1.29 μg/L hydroxyl radicals. The medium diameter sharply declined from 38.6 μm to 17.5 μm, and more porous surface of sludge flocs was observed, indicating that MB-O₃ destroyed the water-oil-gel structure and contributed to the stripping of TPH. The soluble chemical oxygen demand was released by 390% with respect to initial value (from 764 to 3740 mg/L) and acetic acid was the predominant component with yield of 590 ± 7.1 mg/L, which could be served as an additional carbon source. This study provides an efficient approach to achieve sludge disposal and simultaneous enhance the stripping of total petroleum hydrocarbons from oil refinery sludge.

Dinitrodiazophenol industrial wastewater treatment by a sequential ozone Fenton process

The ozonation process is efficient in degrading aromatic substances and substances with unsaturated bonds, but cannot effectively destroy small-molecule organic compounds, which accumulate. Likewise, the Fenton process is a classic wastewater treatment method, but requires strict pH control and produces secondary pollution when the concentration of organic substances is high. In this study, we applied a 1stO₃-2ndFenton sequential process to treat diazodinitrophenol (DDNP) industrial wastewater and provide suitable reaction conditions for Fenton process. For the 1stOzone process, organics removal increased as O₃ dosage increased. At optimized operation, the 1stO₃ process provided an acidic effluent (pH = 3) and reduced the organics concentration to a level suitable for the 2ndFenton process. Benzene ring substances as well as nitro group and diazo group compounds were greatly degraded in the 1stO₃ process and were further mineralized in the 2ndFenton process. Additionally, the biodegradability of DDNP industrial wastewater was greatly improved. This is the first reported time that ozonation and the Fenton process have been integrated sequentially to treat an explosive production wastewater. The study provides a feasible chemical oxidation method for treating DDNP industrial wastewater by simply combining two classic treatment processes.